Increasing demands for data communications have resulted in the development of techniques that provide more cost-effective and efficient means of using communication networks to handle more information, and new types of information. One common technique is to segment the information, which may be a voice or data communication, into packets. A packet is typically a group of binary digits, including both data and control information. Networks, especially multimedia networks, generally carry multiple classes of packet traffic—including voice, image and other framed data. Most networks comprise devices that may source, sink or forward packet-based data. Routing of data is often limited by the processing capacity and physical throughput properties of various functional units and media within the communications network. Such constraints can cause congestion and Quality of Service (“QoS”) problems within the system.
A wireless local area network (WLAN) provides an Ethernet-like channel that uses wireless media (e.g., radio frequency transceivers), instead of wires or cables, to enable data communications between or amongst computers, and other types of electronic devices, on the network. While providing mobility and portability, WLANs avoid the effort and costs involved in running and maintaining cables. WLANs are thus becoming increasingly popular as a communication and networking platform. A WLAN typically has a number of stations and an access point (AP). An AP operates to attach or connect respective stations wirelessly to an external network, and to one another. A WLAN generally has several protocol layers—among which are a physical (PHY) layer, and a medium access control (MAC) sub-layer.
A MAC, usually comprising both hardware and software, is typically located at the AP and at each station—controlling access to the wireless medium (e.g., radio frequency transceivers). Transmission from an AP to a station is referred to as downlink. Transmission from a station to an AP is referred to as an uplink. Finally, transmission between stations is referred to as sidelink.
Often, all transmissions within a WLAN must share a single frequency channel or communication medium. Thus, conflicts or collisions of data packets or frames are likely to occur whenever different traffic streams and bursts simultaneously arrive at a given transmission point, and need to access the shared medium for transport to other transmission points. Accordingly, several approaches have been developed to manage access between competing traffic streams and bursts, and to ensure that allowing access will not cause excessive collisions over the medium.
For example, in one conventional approach, known as Distributed Coordinator Function (DCF), a station must sense the medium before a new data frame is sent from that station. If the medium is found to be idle for at least a DCF Inter Frame Space (DIFS) period of time, the frame is transmitted. Otherwise, a back-off time B—measured in a number of time slots—is chosen randomly from within the interval [0-CW), where CW is the contention window. After the medium has been detected as being idle for at least a DIFS, the back-off timer is decremented for each time slot the medium remains idle. When the back-off timer reaches 0, the frame is transmitted. Upon detection of a collision, a new back-off time is chosen, using a CW of double the previous one, and the back-off procedure starts over. Other conventional mediation schemes for determining access include: an enhancement of DCF, referred to as EDCF; Distributed Fair Scheduling; and Blackburst.
In some conventional WLAN access management procedures, isochronous data streams are provided access time during a succession of time or service periods. When a given station has a new data stream, it sends a stream access request to an access coordinator, located at the AP, together with QoS requirements or parameters for the new data stream. When a coordinator decides to admit a new isochronous data stream to schedule access time, provision must be made to ensure that QoS requirements for previously admitted isochronous streams will still be met.
Thus, conventional systems often employ some sort of worst-case or maximum timing scheme to ensure QoS standards are met. For example, consider a system in which the longest possible arrival of an isochronous stream to be transmitted requires an access time on the order of 10 milliseconds. In a conventional system where each successive service period is on the order of 20 milliseconds, only two streams are admitted for access. This is done to ensure that all admitted streams have sufficient access time. On average, however, some of the admitted streams actually have shorter arrival times, while others actually have longer arrivals in a given service period. Thus, much more than two streams could be admitted and given access times. As a result, a great deal of access time for transporting data across the WLAN is unused, and thus wasted, when admission control is based on the maximum temporal needs of individual streams.
As a result, there is a need for a system for controlling WLAN admission access that was readily adaptable to isochronous streams of widely varying interval and arrival times, overcoming the inefficiencies of conventional access systems. Moreover, it would be desirable to provide such a system for controlling WLAN admission access without degrading or diminishing QoS for respective admitted isochronous streams and asynchronous bursts.